R.J. Carsky, N. Wolo, V.M. Manyong and G. Tian
A simple balance sheet was used to identify nutrient limitations in yam production and to predict changes in soil nutrient stocks as a result of adoption of soil improvement systems for yam, using Gliricidia sepium agroforestry (cut-and carry and in situ) systems as examples. Fixed nutrient inputs taken from the literature include weathering, atmospheric deposition, non-symbiotic. Biological N-fixation (BNF) and those present in the yam seed while fixed exports are leaching and runoff/erosion. Variable inputs are nutrients in G. sepium mulch and crop residue return. Variable exports consist of nutrients in the yam tubers and uptake by trees when grown in the same field with yam. Literature review has provided estimates of yam tuber nutrient concentrations, expected amounts of G. sepium mulch and its nutrient content. Sensitivity analysis using the model suggests that nutrient contents of yam and G. sepium mulch are important terms in the equation and therefore should be locally measured. The model predicts N depletion in an in situ system except for yam yield of 10 Mg/ha or less and G. sepium mulch application of 4.5 Mg/ha or more. Higher yields of yam tubers (15 to 20 Mg/ha) are attainable without soil depletion in a cut-and-carry system with 3 to 4.5 Mg/ha of mulch. Without K fertilizer, the model predicts potassium mining under all scenarios of an in situ system and even m a G. sepium cut-and-carry system unless 3 to 4.5 Mg/ha of mulch is applied. The results suggest that for long-term sustainability, K must he applied either directly to the yam crop or to the associated G. sepium when the two are grown in association.